On 15/03/2019 19:51, Andrey Konovalov wrote:
This patch is a part of a series that extends arm64 kernel ABI to allow to
pass tagged user pointers (with the top byte set to something else other
than 0x00) as syscall arguments.
strncpy_from_user and strnlen_user accept user addresses as arguments, and
do not go through the same path as copy_from_user and others, so here we
need to handle the case of tagged user addresses separately.
Untag user pointers passed to these functions.
Note, that this patch only temporarily untags the pointers to perform
validity checks, but then uses them as is to perform user memory accesses.
Thank you for this new version, looks good to me.
To give a bit of context to the readers, I asked Andrey to make this change, because
it makes a difference with hardware memory tagging. Indeed, in that situation, it is
always preferable to access the memory using the user-provided tag, so that tag
checking can take place; if there is a mismatch, a tag fault will occur (which is
handled in a way similar to a page fault). It is also preferable not to assume that
an untagged user pointer (tag 0x0) bypasses tag checks.
Kevin
Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
---
lib/strncpy_from_user.c | 3 ++-
lib/strnlen_user.c | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
index 58eacd41526c..6209bb9507c7 100644
--- a/lib/strncpy_from_user.c
+++ b/lib/strncpy_from_user.c
@@ -6,6 +6,7 @@
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/errno.h>
+#include <linux/mm.h>
#include <asm/byteorder.h>
#include <asm/word-at-a-time.h>
@@ -107,7 +108,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
return 0;
max_addr = user_addr_max();
- src_addr = (unsigned long)src;
+ src_addr = (unsigned long)untagged_addr(src);
if (likely(src_addr < max_addr)) {
unsigned long max = max_addr - src_addr;
long retval;
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index 1c1a1b0e38a5..8ca3d2ac32ec 100644
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -2,6 +2,7 @@
#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/uaccess.h>
+#include <linux/mm.h>
#include <asm/word-at-a-time.h>
@@ -109,7 +110,7 @@ long strnlen_user(const char __user *str, long count)
return 0;
max_addr = user_addr_max();
- src_addr = (unsigned long)str;
+ src_addr = (unsigned long)untagged_addr(str);
if (likely(src_addr < max_addr)) {
unsigned long max = max_addr - src_addr;
long retval;
